This invention relates to optical control of the coextruded thickness of one or more layers of a multilaminate resinous sheet by relative color difference.
In the past, many different techniques have been employed to measure the thickness of a number of different types of films on a number of different substrates. For example, surface profile monitors have been used to track a stylus over the surface of various substrate materials. However, physical contact can distort or damage the surface; accordingly, methods based on light reflection from the film surface have been employed to offer a non-contact measurement. However, coextruded resinous materials heretofore have not been so measured.
More particularly, various optical methods have been used to measure the thickness of for example, dielectric layers, protective inorganic films, and antireflective coatings on glass as in U.S. Pat. Nos. 2,584,583; 2,845,838; 3,395,278; 3,544,222; 3,773,40; and 3,892,490. Other optical techniques have been adapted to gauge the thickness of organic layers applied to metal substrates as for example U.S. Pat. Nos. 3,017,512 and 4,207,467. Still others monitor the thickness of films and coatings in the paper, printing and pharmaceutical industries as in U.S. Pat. Nos. 2,773,412 and 3,874,794. Some processes are generally applicable to measuring or monitoring the optical thickness of thin films without regard to the type of substrate as in U.S. Pat. Nos. 3,645,623; 4,320,967 and 4,355,903. A few techniques have measured the thickness of transparent plastic films such as U.S. Pat. Nos. 2,655,073; 3,732,016; 3,997,268; 4,285,745; and 4,440,089.
Although specialized monitors employing X-rays or ultraviolet light, or infrared have been developed, there are essentially two distinct techniques for optical determination of film thickness. The first, known as ellipsometry, measures and compares the reflection coefficient measured at a given wavelength and an angle of incidence for polarization perpendicular and parallel to the plane of incidence. Considerable computation is required and unambiguous results are obtained only if it is known a priori that the film thickness lies within a restricted range, usually 0 to 3000 Angtroms. The second class of optical techniques is based on the wavelength and/or angle of incidence dependence of the reflectivity, observable as the well-known interference colors in thin films. Although the principle of this technique is simple, instruments based on this principle are not cheap, even when they do not give a direct reading of the film thickness. These previously known techniques have the further disadvantage in measuring the thickness of transparent and semitransparent resinous films in that they have not been adapted to aid coextrusion processes.
Accordingly, a non-contact, simple, essentially optical technique, devoid of the prior art problems, which can measure the thickness of one or more layers of a coextruded multilaminate resinous material would be a substantial advancement in the art.